Distribution of light produced by a light source can be important or even critical in some applications. The light source can be, for example but not necessarily, a light emitting diode “LED”, a filament lamp, or a gas-discharge lamp. FIG. 1a shows a view of a section taken from an exemplifying light guide 101 according to the prior art for modifying a light distribution pattern of a light source 102. Some of the light beams radiated by the light source 102 are depicted with dashed line arrows in FIG. 1a. The light guide 101 can be rotationally symmetric with respect to a geometric line 120 shown in FIG. 1a. The geometric line 120 is parallel with the z-axis of a coordinate system 199. The light guide 101 is made of transparent material having refractive index greater than unity. The light guide comprises a first end 103 comprising a place for the light source, a second end 104 opposite the first end, and an outer surface 105 between the first and second ends. The outer surface 105 is shaped to provide total reflection for reflecting light towards the second end 104. An inconvenience related to light guides of the kind described above is that also light that is not reflected by the outer surface 105 is attenuated by the transparent material. FIG. 1b shows a view of a section taken from another exemplifying light guide 111 according to the prior art for modifying a light distribution pattern of a light source 112. Some of the light beams radiated by the light source 112 are depicted with dashed line arrows in FIG. 1b. The light guide 111 can be rotationally symmetric with respect to a geometric line 121 shown in FIG. 1b. The light guide 111 is a bowl-like element having a first end 113 comprising a place for the light source 112, a second end 114 opposite the first end, and a reflective layer 115 on the inner surface of the bowl-like element. In this case, light is not attenuated by any transparent material, but an inconvenience related to the light guide illustrated in FIG. 1b is the need for the reflective layer 115 on the inner surface of the bowl-like element.